Came here to see if anyone made a Sion bug thread too, sad that not only his bugs didn't get fixed in 8.2 but it added new ones.
If you fix his Q interruption cooldown, be sure to make sure it works against knockback and knockup skills, it was a longstanding issue that displacement type skills like Naut hook could set it to the full cooldown instead of the reduced one.

I came here to report something similar, Sion E used to prevent a minion from dying until it reached the end of it's travel distance but that's no longer the case this patch, now the travel distance will be cut short if it dies to anything and will prevent his E from being able to last hit if anything else is hitting the minion.
I'm hoping this is a bug that will be fixed because it is quite a disadvantage.

I came here to leave a bug report aswell and saw your thread, I'm a Sion main, play him top/jungle/support on my main and alt accounts and has been a favorite since the rework.
There is a VERY noticeable lag delay between him finishing Q and being able to do anything now, I believe the bug was caused by either one of these in the bugfixes, most likely the second
Fixed a bug where casting Sion's Q - Decimating Smash could remove Spell Shield effects from enemies behind him
Fixed a bug where casting Sion's Q - Decimating Smash right before an basic attack against a turret would cancel the basic attack.
I'm going to have to shelve my main man Sion until this is fixed, it is a huge unintended nerf to his effectiveness and feels very clunky in play to be unable to do anything for half a second longer.
The early numbers from the win % sites are also showing Sion just plummeting after this bug http://current.lolalytics.com/champion/Sion/

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> [{quoted}](name=NemeBro,realm=NA,application-id=3ErqAdtq,discussion-id=frtf0TPO,comment-id=0000,timestamp=2015-08-08T22:45:40.944+0000)
>
> Sion is given poor base tank stats because he has the potential to inflate his health more than any item could by farming (with this health being increased further by cinderhulk). Whether you agree with it or not, this is the reasoning Riot has given.
That, and while Sion's abilities aren't the hardest to avoid, if he _does_ land them, he can two-shot most other thanks at level two with nothing in his build but a cloth armor.

Sion already needs around 250 HP from his health passive to *break even* with other tanks duo to his low base health, I just figure he doesn't need to be forgotten about if other tanks are getting base armor buffs when frozen heart isn't nearly as vital on them as it is on Sion.

I'd like to add that with the 5.9 dragon damage buff, you need to add a extra rank to his shield before it can survive a single hit from dragon and you can't reactivate it in time to explode it before a second attack comes out, which really hurts his dragon tanking and bursting it in a smite contest.

I just feel with a slight bump to the base health of the shield his jungle would be restored, it currently can't tank a hit from a big monster + a small monster on any camp, both krugs will break it and a buff + one of the two mini monsters will break the shield before you can detonate it in time while still absorbing anything.
a extra 20-30 base HP to the shield would be enough for it to tank a single hit from the buff monsters and still be detonated.